The cover plate on the carrying side and that on the running side of a conveyor belt normally consist of a rubber mixture containing a rubber component or a rubber component blend, a wetting agent or a wetting system, comprising a wetting agent and an accelerator, and also normally further mixing ingredients, in particular a filler and/or a processing aid and/or an aging prevention aid and/or a plasticizer and/or other additives (for example, fibers, colored pigments). The relevant rubber basis is in particular:    natural rubber (NR)    butadiene rubber (BR)    chloroprene rubber (CR)    styrene-butadiene rubber (SBR)    nitrile rubber (NBR)    butyl rubber (IIR)    ethylene-propylene rubber (EPM)    ethylene-propylene-diene rubber (EPDM)    SBR/NR blend    SBR/BR blend    NR/BR blend.
Of particular importance hitherto has been CR, which is distinguished by high resistance to flames, weathering and aging, in particular for conveyor belts with use in underground mining. Furthermore, NR and the aforementioned blends (DE 10 2009 043 904 A1) have acquired greater importance in surface mining.
As a result of the vulcanization of a rubber mixture of the aforementioned type, the conveyor belt is imparted the required elastic properties.
The conveyor belt is provided with embedded tension members or reinforcement. The embedded tension members used in the longitudinal direction of the conveyor belt are cords made of steel or aramid, cords made of steel being of particular importance. However, the tension member can also be a single-layer or multi-layer textile structure, in particular in the form of a woven fabric. A polyamide-polyester fabric is of particular importance here. In particular in conjunction with steel cord conveyor belts, for the purpose of slit protection, an embedded transverse reinforcement which is embedded in the cover plate on the carrying side and/or on the running side and is made of synthetic cords, for example of polyamide, is additionally used (WO 2008/034483 A1).
At least one conductor loop is fully embedded in the cover plate on the carrying side and/or on the running side of the conveyor belt. The conductor loop is a continuous (short-circuited) loop. Normally, a plurality of conductor loops are embedded in a conveyor belt at a spacing of 50 to 200 m, in particular 50 to 100 m, to be specific as individual conductor loops or a packet of conductor loops.
With regard to conductor loop technology, reference is made in particular to the following patent literature:    DE 15 56 723 A1    DE 40 14 475 A1    DE 44 44 264 C1    DE 196 07 867 A1    DE 198 27 120 A1    U.S. Pat. No. 6,581,755    DE 101 00 249 A1    U.S. Pat. No. 4,621,727    U.S. Pat. No. 7,178,663    US 2012/0012444    AU 57 558 B    GB 1 246 786 A
Conveyor belt conductor loop technology will be explained in more detail below.
The conductor loop has the task of making the occurrence of longitudinal slits detectable. Here, the conductor loop is checked for intactness with the aid of an interrogation station in the form of a transmitter/receiver pair. If a longitudinal slit occurs in the conveyor belt, the conductor loop will also be destroyed and the transmitter/receiver transmission link will be interrupted. A criterion for stopping a conveyor belt at the start of the slitting process and limiting the length of the slit is therefore obtained. As a result, the maximum possible length of the slit is limited to the distance between two conductor loops plus the braking travel. Unfortunately, after relatively long use of the conveyor belts, false alarms, which are triggered by the fact that the conductor loop has been destroyed without a slit in the belt having occurred, occur more and more frequently.
The conductor loop frequently consists of meander-shaped built-in metal cords, in particular of steel cords. The cords in turn normally comprise at least five strands. Preference is given here to a cord structure of the 7×7 type with an individual wire diameter of about 0.2 mm and an overall diameter of 1.5 mm to 2 mm. There are also variants which comprise a mixed structure of copper strands and steel strands, wherein, in particular, the steel strands encase the copper strands. Also known is a version consisting of a solid copper wire encased in steel strands.
The meander-shaped structure (DE 196 07 867 A1, FIG. 1) is particularly suitable for increasing the extensibility of the conductor loop, since the latter would otherwise be destroyed after a relatively short time by the mechanical stress in the conveyor belt, namely bending and loading stress. The production of the meanders requires the use of a relatively ductile material but this has a detrimental effect on the service life of the conductor loop. Although materials with a higher elasticity improve the long-term strength of the individual cord considerably, they make it more difficult to form this meander. In addition, the use of so-called “high-elongation” cords (HE cords), which, as a result of their structure, are more extensible than the cords, improves the service life but likewise makes forming the meanders more difficult.
In DE 101 00 249 A1, a conductor loop is presented which has a higher cord mass within the edge region of the conveyor belt than in the middle region of the conveyor belt. The conductor belt here is also free of crossing points, specifically as opposed to the 8-shaped conductor loop, as described in U.S. Pat. No. 4,621,727. By using such a design, it is possible to achieve a higher electromagnetic sensitivity.
U.S. Pat. No. 7,178,663 discusses a more recent conductor loop development. Here, in order to provide a highly flexible and highly extensible conductor loop with an increased service life, it is proposed to form the conductor loop as an open stranded helix, each individual wire being encapsulated by a polymer material with elastic properties. As a result of this measure, in addition the internal friction between the wires is suppressed.
The disadvantages of conductor loops according to the prior art listed in more detail here, according to which the conductor loop cords normally consist of steel, are their susceptibility to corrosion, their low flexibility, despite the measures according to U.S. Pat. No. 7,178,663, their low extensibility and lastly the susceptible connection within the context of the spliced belt.
In United States patent application publication 2012/0012444, for the purpose of increasing the flexibility with simultaneous material resistance, a conductor loop is now presented which is formed from at least one hybrid filament. The hybrid filament, in turn, consists at least of a textile first material and a conductive second material.
The textile first material is preferably a polyamide (PA), aramid, polyester (PES), polyvinyl acetal (PVA), polyimide (PI), polyether ether ketone (PEEK) or a polyphenylene. From the group of polyphenylenes, a polyphenylene sulfide (PPS) is significant.
The conductive second material is in particular a corrosion-resistant metal. Of particular importance is a noble metal, in particular in the form of silver, in particular, in turn, as pure silver.
The textile first material normally forms the core and the conductive second material the sheath of the hybrid filament. The encapsulation is carried out by electroplating or electrochemically.
Mostly, at least two hybrid filaments form a composite filament, specifically in the form of a cord or thread. The composite filament here has a thickness of 0.5 mm to 2 mm.
With regard to the subject concerning the hybrid conductor loop, reference is additionally in particular further made to DE 15 56 723 A1 and GB 1 246 786 A which are incorporated herein by reference.